Engines, for example IC engines burning gasoline, diesel or biofuel, output various harmful substances which must be treated to meet current and future emissions legislation. Most commonly those substances comprise hydrocarbons (HC), carbon monoxides (CO), mono-nitrogen oxides (NOx) and particulate matter, such as carbon (C), a constituent of soot. Some of those substances may be reduced by careful control of the operating conditions of the engine, but usually it is necessary to provide apparatus, such as an emissions cleaning module, downstream of the engine to treat at least some of those substances entrained in the exhaust fluid. Various apparatus for reducing and/or eliminating constituents in emissions are known. For example, it is known to provide an oxidation device, such as a diesel oxidation catalyst, to reduce or to eliminate hydrocarbons (HC) and/or carbon monoxide (CO). Oxidation devices generally include a catalyst to convert those substances into carbon dioxide and water, which are significantly less harmful. As a further example, emissions cleaning modules may include a particulate filter to restrict the particulates present in the exhaust gas from being output to atmosphere.
By use of an emissions cleaning module, engine emissions can be cleaned, meaning that a proportion of the harmful substances which would otherwise be released to atmosphere are instead converted to carbon dioxide (CO2), nitrogen (N2) and water (H2O).
In addition, it is known to reduce or eliminate mono-nitrogen oxides (NOX) in diesel combustion emissions by conversion to diatomic nitrogen (N2) and water (H2O) by catalytic reaction with chemicals such as ammonia (NH3) entrained in the exhaust gas.
Generally ammonia is not present in exhaust fluids and must therefore be introduced upstream of a catalyst, typically by injecting a urea solution into the exhaust gas which decomposes into ammonia at sufficiently high temperatures.
By these methods, engine fluids can be cleaned, meaning that a proportion of the harmful substances which would otherwise be released to atmosphere are instead converted to carbon dioxide (CO2), nitrogen (N2) and water (H2O).
Exhaust systems, which may include an emissions cleaning module, may therefore comprise an injector module for injecting a fluid, such as urea, into the exhaust fluid flow. It is also known to include an exhaust mixer to aid mixing of the injected urea with the exhaust fluid flow. For example, WO2011/062960 describes an exhaust mixer provided for use in an engine exhaust system, wherein the exhaust mixer comprises a plurality of vanes arranged in an equally spaced array around a central axis. The exhaust mixer acts to create turbulence within the exhaust flow with an aim of improving the mixing of the urea with the exhaust fluid flow.
Against this background there is provided an emissions cleaning module comprising an improved arrangement of exhaust mixer, an improved engine exhaust system comprising an exhaust mixer and an improved method of mixing an additive into an exhaust fluid.